Mental health studies provide correlational evidence for a predisposition for separation anxiety disorders in children of parents with panic disorders, and for developmental stability of behaviors associated with anxiety in children. Pharmacologic studies have found ultrasonic vocalization (USV) rates of infant rats in response to isolation to be a measure of an early anxiety-like state. To examine processes underlying generational and developmental influences on anxiety, the investigator's laboratory has produced lines of rats (N:NIH strain) selectively bred on the basis of differences in USV responses to isolation at 10 days of age (High, Low and Random lines). In the fourth selected (S4) generation USV rates in the three lines have diverged significantly in expected directions. This is a unique model, the first to accomplish selection for an infantile trait, and it raises fundamental developmental, evolutionary and physiological questions. The model will be tested with respect to three specific aims. Aim 1. To characterize the developmental course of the USV response to isolation in the selected line. The study will combine cross-sectional and longitudinal designs to test USV rates at postnatal days 3, 10, 14, and 18, to determine whether selection for USV at 10 days of age has altered the rate of development of USV in High and Low lines. Aim 2. To determine whether USV response differences between lines are present under different eliciting conditions. Studies will measure USV: a) in response to a stimulus other than isolation that elicits USV; b) in response to different durations of isolation; c) in response to ambient temperatures outside the range at which lines were selected. Aim 3. To determine the extent to which line differences in physiological systems have emerged during selection. Studies will include measures of: a) plasma adrenocorticotrophic (ACTH) hormone response; and b) autonomic and cardiac rate responses to isolation; both systems are involved in the reaction to isolation in pups and adult stress responses.